2019 A Level H2 Chemistry Paper 1 Question 27 - Deduce Formula of Chromium Complex in Excess Hydroxide

Let's take a look at 2019 A Level H2 Chemistry Paper 1 Question 27.

2019 p1 q27 qn

We are required to deduce the formula of the chromium containing species in the deep green solution.

The starting chromium complex in chromium (III) chloride should be [Cr(H2O)6]3+ where chromium (III) ion is surrounded by water ligands.

2019 p1 q27 excess hydroxide added to aqueous chromium III ion

When hydroxide is added, the precipitate formed should be [Cr(H2O)3(OH)3] due to ligand exchange between H2O and OH-.

When more OH- is added, the remaining H2O ligands will be substituted with OH- to form [Cr(OH)6]3- complex which should give us the deep green solution.

Now we can go through the options.

A. [CrCl4]2-

This complex is not formed here but we can get this complex if concentrated HCl is added to provide a high concentration of Cl- ligands.

B. [Cr(H2O)6]3+

This is our starting complex so is not the answer.

C. [Cr(OH)6]3-

This is the complex formed on adding excess OH- ligands so this will be the answer to this question.

D. [Cr(OH)3(H2O)3]

This is the initial precipitate formed hence is not the answer.

Topic: Transition Elements, Inorganic Chemistry, A Level Chemistry, Singapore

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